Refractory metals are members of a group of elements that are difficult to isolate in pure form because of the stability of their compounds, such as oxides, chlorides and fluorides. Since the manufacturing of refractory metals is very complex, we will use tantalum extractive metallurgy as an example to illustrate the development of this technology.
State of the art tantalum powder production is based on the process of reducing potassium heptafluorotantalate (K2TaF7) with sodium (sodium reduction). The modern method for manufacturing tantalum was developed by Hellier and Martin (U.S. Pat. No. 2,950,185). A molten mixture of K2TaF7 and a diluent salt, typically NaCl, KF and/or KCl, is reduced with molten sodium in a stirred reactor. The manufacturing process requires the removal of the solid reaction products from the retort, separation of the tantalum powder from the salts by leaching with dilute mineral acid, and treatments like agglomeration and deoxidation to achieve specific physical and chemical properties. While the reduction of K2TaF7 with sodium has allowed the industry to make high performance, high quality tantalum powders primarily used in solid tantalum capacitor manufacturing; there are several drawbacks to this method. It is a batch process prone to the inherent variability in the system; as a result, batch-to-batch consistency is difficult. Using diluent salts adversely impacts the throughput. The removal of chlorides and fluorides in large quantities presents an environmental issue. Of fundamental significance, the process has evolved to a state of maturity such that a significant advance in the performance of the tantalum powder produced is unlikely.
Over the years, numerous attempts were made to develop alternate ways for reducing tantalum compounds to the metallic state (U.S. Pat. Nos. 1,602,542; 1,728,941; 2,516,863; 3,647,420; and 5,356,120). Among these was the use of active metals other than sodium, such as calcium, magnesium, and aluminum, and raw materials such as tantalum pentoxide and tantalum chloride.
Kametani et al. (GB 2231883) developed a process for reducing gaseous titanium tetrachloride with atomized molten magnesium or sodium in a vertical type reactor in the temperature range of 650-900° C. Though the reaction was very exothermic, it was not self-sustaining due to a special effort designed to avoid the formation of titanium-iron intermetallic compounds at high temperatures (the melting point of Fe—Ti eutectic is 1080°).
U.S. Pat. Nos. 1,602,542, 3,658,507 and 2,881,067 suggest the use of gaseous magnesium to better control the process parameters. The gaseous reducing agent was generated in-situ from a mixture of metal oxide and reducing agent, or outside the reactor enclosure. Patentees managed to produce at bench scale fine zirconium, titanium, tungsten, molybdenum and chromium powders. The method was of batch type. The only controlled parameter was the magnesium (calcium) partial pressure. The kinetics and the temperature of the charge were a function of the gaseous magnesium (calcium) flow rate and were impossible to control due to the condensation of magnesium (calcium) on the cold parts of the reactor. Since both melting and evaporation of Mg (Ca) without condensation on the cold parts was practically impossible, the process had to be periodically stopped for the removal of the buildup. Therefore, continuous operation could not be carried out.
Numerous attempts have been made to produce tantalum and niobium powders by metalothermic reduction of their oxides with Mg, Al or Ca in a bomb type reactor (U.S. Pat. Nos. 1,728,941 and 2,516,863). A blend of finely-divided oxide and metal reducing agent was placed into a reactor and then ignited. The temperature could not be controlled and therefore it was not possible to achieve reproducible physical and chemical properties of the metal powders. The residual Mg (Al, Ca) content was high due to the formation of tantalates and niobates. The process was found to be unsuitable for manufacturing high quality capacitor grade powders.
Shekhter et al. (U.S. Pat. No. 6,171,363) described a method for controlled reduction of tantalum and niobium oxide with gaseous magnesium to produce capacitor grade tantalum and niobium powders (batch magnesium reduction). The key is control of the reaction process to achieve essentially isothermal conditions. The batch magnesium reduction process requires excess amount of magnesium to compensate for its condensation on the cold parts of the furnace.
The process disclosed by Shekhter et al. was advantageous compared to the traditional sodium reduction process. For example, there are no fluorine bearing compounds and there is no need to use any diluent salt.
U.S. Patent Application Publication Nos. 2002/0066338 and 2004/0163491, both to Shekhter et al., disclose a method of making high purity refractory metals suitable for use in electrical, optical and mill product/fabricated parts produced from their respective oxides by metalothermic reduction of a solid or liquid form of such oxide using a reducing agent selected from magnesium, calcium, and aluminum that establishes (after ignition) a highly exothermic reaction, the reaction preferably taking place in a continuously or step-wise moving oxide such as gravity fall with metal retrievable at the bottom and an oxide of the reducing agent being removable by leaching or in other convenient form and unreacted reducing agent derivatives being removable by leaching or like process.
Unlike metal after sodium reduction, the magnesium reduced powders contain tangible amounts of magnesium after magnesium reduction. Depending on the reduction conditions used, i.e., excess Mg, temperature, residence time, oxide/magnesium particle size, etc., the magnesium content in the powder can vary from 0.02 to 7% by weight.
According to X-ray diffraction analysis in tantalum/niobium powders, the magnesium does not exist in the elemental form but forms complex oxide compounds referred to as magnesium tantalate/niobates. In particular, the X-ray diffraction pattern was used to identify the chemical formula of a particular oxide present as Mg4Ta2O9. Tangible quantities of magnesium tantalate/niobates can adversely affect physical, chemical, and electrical properties, thus, the conversion of magnesium tantalate/niobates into metals is an important issue.
It is a principle object of the present invention to provide a new process for producing high performance, high quality tantalum, niobium, and other refractory metals and blends or alloys thereof by reducing solid/liquid metal oxides in a steady, self-sustaining reaction zone, thereby eliminating one or more, and preferably all, of the problems associated with the traditional double salt reduction and other processes described above, while minimizing or eliminating the presence of magnesium tantalate/niobates.